Cellulose food casings are well known in the art and are widely used in the production of stuffed food products such as sausages and the like. Cellulose food casings generally are seamless tubes formed of a regenerated cellulose and contain a plasticizer such as water and/or a polyol such as glycerine. Plasticization is necessary because otherwise the cellulose tube is too brittle for handling and commercial use.
Cellulose food casings generally are used in one of two forms. In one form the casing consists of a tubular film of pure regenerated cellulose having a wall thickness ranging from about 0.025 mm to about 0.076 mm and made in tube diameters of about 14.5 mm to 203.2 mm. The second form is a reinforced casing wherein the tubular wall of the casing consists of a regenerated cellulose bonded to a paper web. Such reinforced casings are commonly called "fibrous" casings to distinguish them from the nonreinforced cellulose casings. Fibrous casings have a wall in the range of 0.050 mm to 0.102 mm thick and are made in diameters of about 40.6 mm to 193 mm or greater.
The cellulose for making both types of casings is most commonly produced by the so called "viscose process" wherein viscose, a soluble cellulose derivative, is extruded as a tubular film through an annular die into coagulating and regenerating baths to produce a tube of regenerated cellulose. This tube is subsequently washed, plasticized with glycerine or other polyol, and dried. Drying usually is accomplished while the tube is inflated with air at a pressure sufficient both to maintain a constant tube diameter and to orient the film.
The viscose process for making cellulose is well known in the art. Briefly, in the viscose process a natural cellulose such as wood pulp or cotton linters first is treated with a caustic solution to activate the cellulose to permit derivatization and extract certain alkali soluble fractions from the natural cellulose. The resulting alkali cellulose is shredded, aged and treated with carbon disulfide to form cellulose xanthate which is a cellulose derivative. The cellulose xanthate is dissolved in a weak caustic solution. The resulting solution or "viscose" is ripened, filtered, deaerated and extruded. The pulp source and time of aging the alkali cellulose are selected depending upon whether the viscose will be used to make fibrous casing or nonreinforced cellulose casing. For fibrous casing the selection is such that a less viscous solution is used. The lower viscosity solution wicks into the paper web allowing complete penetration of the viscose for establishing strong intercellulose bonding. For extrusion of a nonreinforced cellulose casing, the selection is such that a more viscous, solution is used.
The viscose is extruded as a tube through an annular die and about a self centering mandrel into coagulation and regenerating baths containing salts and sulfuric acid. In the acidic baths the cellulose xanthate, e.g. viscose, is converted back to cellulose. In this respect, the acid bath decomposes the cellulose xanthate with the result that a pure form of cellulose is coagulated and regenerated. Initially, the coagulated and regenerated cellulose is in a gel state. In this gel state the cellulose tube first is run through a series of rinse water dip tanks to remove by-products formed during regeneration. The gel tube then is treated with a glycerine humectant and dried to about 10% moisture based on total casing weight. As noted above, the gel tube is inflated during the drying process to a pressure sufficient to provide a degree of orientation to the dried cellulose tube.
Both nonreinforced cellulose casings and fibrous casings are produced in this fashion except in the case of fibrous casings the viscose is extruded onto a tube of paper prior to entering the coagulation and regenerating baths.
During regeneration of the cellulose from the xanthate solution, sulfur products are liberated and gases such as hydrogen sulfide, carbon disulfide and carbon dioxide are released through both the inner and outer surfaces of the gel tube. It should be appreciated that the gases produced as by products during regeneration are noxious and toxic so their containment and recovery imposes a considerable burden on the manufacturing process. Moreover, gases generated at the internal surface of the extruded tube can accumulate within the tubular casing and consequently present special problems. The tubular casing while in its gel state is expansible and the pressure build up of gases accumulating within the gel casing causes undesirable diameter variations. To prevent this, the gel casing is punctured periodically to vent the accumulated gases. This puncturing process, involving procedures to puncture, vent, and then seal the punctured gel tube, results in an undesirable interruption of the manufacturing process. Also, gases which evolve within the casing wall may become entrapped causing bubbles which weaken the casing and detract from its stuffability.
Also, the casing in its gel state to some extent retains low residual levels of the sulfur compounds produced during regeneration. While care is taken to remove all residual sulfur compounds by washing the gel tube prior to drying, the dried casing may still contain trace amounts of these compounds.
Despite the problems inherent with the viscose process as described above, it nevertheless remains today as the most commonly used process for the production of cellulose casing for the food processing industry.
An alternate cellulose production method involves forming a cellulose solution by means of a simple dissolution rather than requiring prior derivatization to form a soluble substance (as in the viscose process). A cellulose dissolution process is described in U.S. Pat. No. 2,179,181. This patent discloses the dissolution of natural cellulose by a tertiary amine oxide to produce solutions of relatively low solids content, for example 7 to 10% by weight cellulose dissolved in 93 to 90% by weight of the tertiary amine oxide. The cellulose in the resulting solution is nonderivatized prior to dissolution. U.S. Pat. No. 3,447,939 discloses use of N-methyl-morpholine-N-oxide (NMMO) as the cyclic amine oxide solvent wherein the resulting solutions, while having a low solids content, nevertheless can be used in chemical reactions involving the dissolved compound, or to precipitate the cellulose to form a film or filament.
More recent patents such as U.S. Pat. Nos. 4,145,532 and 4,426,288 improve upon the teachings of the '939 Patent. U.S. Pat. No. 4,145,532 discloses a process for making a solution of cellulose in a tertiary amine oxide such as NMMO which contains 10-35% by weight of cellulose. This higher solids content, achieved in part by including an amount of water (from 1.4% to about 29% by weight) in the tertiary amine oxide solvent, provides a solution adapted for shaping into a cellulosic article such as by extrusion or spinning. In U.S. Pat. No. 4,426,288 the NMMO-cellulose solution contains an additive which reduces decomposition of the cellulose polymer chain so that molding or spinning substances are obtained with only slight discoloration and which will yield molded shapes distinguished by improved strengths upon precipitation in a nonsolvent such as water.
Using NMMO as a solvent for cellulose eliminates the need for derivatizing the cellulose, as in the viscose process. Consequently it eliminates the disadvantages attendant to the viscose process such as the problems associated with the generation of toxic and noxious gases and sulfur compounds.
However, while nonderivatized cellulose resulting from the process of dissolving cellulose in NMMO eliminates certain problems associated with the viscose process, to applicant's knowledge, NMMO-cellulose solutions heretofore have not been used in the manufacture of cellulose food casings. This perhaps is due in part to the fact that the nonderivatized cellulose solution is thermoplastic with a melting point of about 65.degree. C. so it is normally solid at the temperature heretofore used in the extrusion of viscose (e.g. cellulose xanthate) for producing cellulose food casings. Also, the nonderivatized cellulose solution at 65.degree. C. has a viscosity significantly higher than the viscosity of the derivatized cellulose heretofore used in the production of cellulose food casings. In particular, nonderivatized cellulose in solution may have a molecular weight of about 80,000 to 150,000 and a viscosity in the range of about 60,000 to 100,000 centipoises. The high molecular weight and viscosity is because the dissolution of the cellulose does not affect the degree of polymerization. Viscose for casing manufacture (wherein the degree of polymerization is affected by derivatization) has a molecular weight in the range of about 80,000 to 100,000 for fibrous casing and about 95,000 to 115,000 for nonfibrous casing and a viscosity of 5,000 to 30,000 centipoises.
Accordingly, it is a principle object of the present invention to provide a cellulosic food casing article and method which eliminates the need to derivatize the cellulose used in forming the food casing.